variable vane turbocharger

In my last post I think I have a lemon I asked about the lack of power. I have done a load of checks and I have found that the vacuum diaphragm unit is not building up any presure. On the top of the diapragm is a black pipe in the past when I took this off there would be lots of suction but today there is none. I have checked all the pipes coming away from it all does seem to be fine. Could anyone tell me if the diaphragm could be gone and if so can I change just this or do I need to by a new turbo. I have no black smoke and the car still travels at motorway speeds it just takes awhile to get there. Thanks to all.